Feline Leukemia Virus (FeLV)



Feline leukemia virus, a retrovirus, is a common infection of cats. It is the cause of more cat deaths, directly or indirectly, than any other organism and is widespread in the cat population

Disease Transmission

Feline leukemia virus infection (FeLV) can be transmitted in several ways:

  1. by the saliva of infected cats contaminating the eye, mouth, and nose membranes of non-infected cats via licking.
  2. by passing infected blood to non-infected cats.
  3. from mother to fetuses (developing kittens) during pregnancy.

Kittens are much more susceptible to FeLV infection than are adult cats and therefore are at the greatest risk of infection if exposed. However, even healthy adult cats can become infected if sufficiently exposed.

Catsy Mitchell DECEMBER 2018 POTM


Most infected cats eliminate the virus and become immune. In those cats that do not develop immunity, the virus spreads to the bone marrow.

Proliferative and degenerative diseases may occur in any of the tissues invaded by the virus, or the virus may be indirectly responsible for other illnesses because of its immunosuppressive effect. A large percentage of the cats that are exposed to the virus will have latent (hidden) infections and will be capable of transmitting the disease in saliva, tears, and urine. Some of these latent carriers will become clinically ill when stressed.

Clinical Signs
FeLV adversely affects a cat's body in many ways. It is the most common cause of cancer in cats, may cause various blood disorders, and may lead to a state of immune deficiency that hinders a cat's ability to protect itself against other infections. Because of this, common bacteria, viruses, protozoa, and fungi that usually do not affect healthy cats can cause severe illness in FeLV-infected cats. These secondary infections are responsible for many of the diseases associated with FeLV.

During the early stages of infection, it is common for cats to exhibit no signs of disease at all. Over time, however, (weeks, months, or even years) an infected cat's health may progressively deteriorate or he/she may experience repeating cycles of illness and relative health. Signs can include:

  • Loss of appetitepet doctor
  • Progressive weight loss
  • Poor coat condition
  • Enlarged lymph nodes
  • Persistent fever
  • Pale gums and other mucous membranes
  • Inflammation of the gums (gingivitis) and mouth (stomatitis)
  • Infections of the skin, urinary bladder, and upper respiratory tract
  • Persistent diarrhea
  • Seizures, behavior changes, and other neurological disorders
  • A variety of eye conditions
  • Abortion of kittens or other reproductive failures

Diagnostic Tests

Necessary diagnostic tests may include blood chemistry, hematology, radiography, bone marrow aspiration, ophthalmoscopy, and specialized antibody tests.


There is no effective treatment for the myeloproliferative (bone marrow) form of leukemia. Treatment is mainly supportive and may require blood transfusions, prednisone, and anabolic steroids.

FeLV cancer (lymphoma) has a better response to therapy than myeloproliferative diseases do. Treatment may include chemotherapy, glucocorticoids, interferon, Protein A, and supportive treatment.


Eighty-five percent of cats with FeLV infection die within 3 years of the diagnosis. 

Prevention Of FeLV

There are several preventive measures that can be taken to decrease the risk of contracting FeLV.

  • Cats can be FeLV tested, and then vaccinated if they are negative. Vaccination is recommended for high-risk cats only. FeLV vaccination of infected cats does not affect the carrier state, the capacity to infect other cats or the development of disease in the infected cats. Vaccination may also be associated with adverse events. (Duration of immunity may vary from fifteen weeks to three years.)
  • Kittens may be tested at any age. However, infection in newborn kittens may not be detected until weeks to months after birth. Therefore, several FeLV tests during the first six months of life may be necessary to feel completely "safe" about a negative test result.
  • All kittens or adult cats that test negative by the first ELISA screening test - but with a known or suspected exposure to FeLV - should be retested. This is done to rule out possible negative results obtained during incubation of the FeLV virus. Although the majority of cats will test positive within several weeks, the final retest of negative cats should be no sooner than 90 days post-exposure.
  • In large catteries, a test and removal program can be instituted.
  • Multi-cat households with FeLV positive cats should be maintained as a closed colony. (No new cats should be brought into the household, to prevent the spread of infection to the new arrivals.)

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A relatively effective vaccine against FeLV is available, although it will not protect 100% of cats vaccinated, and it is not considered a core vaccine. Owners contemplating FeLV vaccination for their uninfected cats should consider the cats' risk of exposure to FeLV-infected cats and discuss the advantages and disadvantages of vaccination with a veterinarian. Since not all vaccinated cats will be protected by vaccination, preventing exposure remains important even for vaccinated pets. FeLV vaccines will not cause false positive FeLV results on ELISA, IFA, or any other available FeLV tests.


Retroviruses are unstable, live for only minutes outside the cat's body, and are readily destroyed by most disinfectants.


Because the feline leukemia virus is so unstable, a new, healthy cat can be brought safely into a "contaminated" house within days of the departure of a FeLV infected cat. Contact us to make an appointment to discuss your cat's risks and consideration for testings and vaccinating your cat to protect him from this disease.

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